netdev - Re: sendmsg() and vlan tags

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Date:	Wed, 31 Jul 2013 14:01:35 -0700
From:	Ani Sinha <ani@...stanetworks.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: sendmsg() and vlan tags

On Tue, Jul 30, 2013 at 5:56 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Tue, 2013-07-30 at 17:29 -0700, Ani Sinha wrote:
>> hello folks.
>>
>> Can anyone please shed lights on this?
>>
>> thanks,
>> ani
>>
>>
>> On Fri, Jul 26, 2013 at 2:30 PM, Ani Sinha <ani@...stanetworks.com> wrote:
>> > Hi folks,
>> >
>> > We know that (since Linux 3.0) on raw sockets, the recvmsg() api
>> > returns the vlan tag information of the packet in the packet aux data.
>> > I looked at the kernel code and it seems on the TX side, in sendmsg(),
>> > we do not accept the vlan tag data from userland for a raw packet. So
>> > if the userland sends a raw packet using sendmsg() with vlan tag
>> > information in aux data, it will be discarded.
>
>
> What is the vlan tag information value in aux data ?
>

It populates tp_vlan_tci field in the aux data (tpacket_auxdata
structure). Specifically, it gets the packet vlan tag ID and vlan
priority fields (PCP) from the skb.

In net/packet/af_packet.c :


a3bcc23e        (Ben Greear     2011-06-01 06:49:10 +0000       2822)
         if (vlan_tx_tag_present(skb)) {
a3bcc23e        (Ben Greear     2011-06-01 06:49:10 +0000       2823)
                 aux.tp_vlan_tci = vlan_tx_tag_get(skb);
a3bcc23e        (Ben Greear     2011-06-01 06:49:10 +0000       2824)
                 aux.tp_status |= TP_STATUS_VLAN_VALID;
a3bcc23e        (Ben Greear     2011-06-01 06:49:10 +0000       2825)
         } else {
a3bcc23e        (Ben Greear     2011-06-01 06:49:10 +0000       2826)
                 aux.tp_vlan_tci = 0;
a3bcc23e        (Ben Greear     2011-06-01 06:49:10 +0000       2827)
         }


The question is, on TX, can the user populate these values in the aux
data of a raw packet and do a sendmsg(). Will the kernel retain these
values. It looks like it does not.

-- 
Ani
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